This invention relates to optical devices and in particular, but not exclusively, to those including a high numerical aperture (NA) optical fibre waveguide.
There are many optical devices in which it is necessary to couple the electromagnetic radiation propagated along an optical waveguide to a bulk optic system. This is often achieved by simply end-firing the radiation from the end of the waveguide, for example from the end of a cleaved optical fibre waveguide.
It should be noted that by xe2x80x9copticalxe2x80x9d is meant that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
The present invention will be discussed in the context of optical fibre waveguides but, as will be clear, the present invention is applicable to other types of waveguide in which there is a waveguiding core surrounded by a cladding-type region, for example buried doped-silica in silica waveguides.
It is often desirable to reduce the reflectivity of the waveguide end to radiation impinging on it via the waveguide. Three currently known ways to do so are to apply an anti-reflection coating to the cleaved surface, to angle the end of the fibre so the end face is not normal to the waveguide, and to use an index-matching compound to fill the gap between the end of the optical fibre and the next optical component.
One application where a low reflectivity transition between guided and bulk optics is required is in coupling the output of an optical fibre amplifier to an optical receiver assembly of which the nearest component to the optical fibre is, perhaps, a radiation gathering lens. The gain provided by such amplifiers can be limited by the reflectivity of the end face. The amplifier will lase if the round trip gain of the amplifier, of which one component is residual facet reflectivity, is greater than the losses.
Index matching compounds are only useful if the next component abuts the optical waveguide, such as a GRIN (Graded Index) lens or plano-lens. Also, there are concerns over the use of such organic materials in the optical path because of their degradation in time by radiation, moisture and other environmental influences.
Multi-layer dielectric anti-reflection coatings are used but it is difficult to reduce the residual reflectivity of the fibre end below 0.1% (30 dB) using such anti-reflection coatings and the lower the reflectance the narrower the wavelength range within which the reflectance is suitably low.
Angling the end of the fibre is useful for low NA fibres (NAxcx9c0.12) where an angle of 12xc2x0 between the cleave and the waveguide axis reduces the reflectance to 40 dB and where the polarisation sensitivity introduced by the angled interface, 0.02 dB for a 12xc2x0 angle, can be accepted. However, for high NA fibres (NAxcx9c0.4), of which optical fibre amplifiers are an example, a 12xc2x0 cleave angle provides an improvement to 20 dB, only a small improvement on 14 dB for a 0xc2x0 facet, while in instrumentation applications a variation in transmitted power with polarisation state of 0.02 dB, or even 0.008 dB for an 8xc2x0 angle, is not acceptable.
According to a first aspect of the present invention there is provided an optical device having an optical radiation output, the optical device comprising an optical waveguide having an end through which the optical radiation output of the device may pass, and a block fusion spliced to the end of the optical waveguide to reduce reflections of the optical radiation output from the said end, wherein the block has a refractive index approximately equal to the effective refractive index of the waveguide, a length such that substantially all the radiation exiting the waveguide propagates directly to, and subsequently through, the free end of the block remote from the waveguide, and the free end of the block is substantially planar.
The present invention provides an optical device in which the waveguide termination can readily have a reflectance of less than 30 dB.
The block is preferably a cylinder of material of dimension and form that can be fusion spliced to the waveguide with a standard fusion splicer. Although the diameter of such a block may be less than the diameter of the waveguide it is preferable that it is of a greater diameter when the waveguide is an optical fibre so that a longer length of block may be used which gives a greater improvement in reflectance.
The end of the block remote from the optical waveguide is preferably provided with an anti-reflection coating. As will be explained in more detail below, a relatively inefficient coating of 0.3%, but one which is broadband and readily applied, can reduce the reflectance from 44 dB to 55 dB which is entirely satisfactory for use in fibre amplifier applications.
Some of the radiation reflected from the internal surface of the end of the block remote from the waveguide can be internally reflected from the sides of the block to re-enter the waveguide so increasing the effective reflectance of the fibre termination. This can be reduced, in a preferred embodiment, by providing the surfaces of the block other than the end face with a coating of a higher refractive index than the block to suppress the above-described internal reflections.
The block should not be too long as this will produce some internal reflections of the radiation as it propagates to the distant end of the block (or is absorbed by the higher refractive coating on the lateral surfaces of the block if provided) and cause a reduction in the radiation that can be output to the external components. If the block is too short the reflectance is higher than the best achievable as will be explained in more detail below. Although such reduced performance may be acceptable, and will fall within the scope of the present invention in its broadest aspects, it is preferable that the core radiation end-firing from the waveguide reaches the distant end face of the block with the base of the cone just within the circumference of that end face so that all, not just substantially all, the radiation propagates directly to the end of the block.
More particularly, if the block is a cylinder of refractive index n, of circular cross-section of radius r and has a length l, and it is substantially coaxial with a waveguide of numerical aperture denoted NA, then the half angle xcex8 of the cone of end-fired radiation equals sinxe2x88x921(NA/n) which is preferably equal to or less than tanxe2x88x921(r/l).
Although applicable to optical fibre waveguides generally, the present invention finds particular application with optical fibre waveguides of high NA, i.e.  greater than 0.15, for example in the region of NA=0.4. An erbium fibre optical amplifier is an example of such a high NA fibre for which the present invention is particularly useful.
In the case of a silica-based, optical fibre waveguide, such as an erbium fibre amplifier, the block is conveniently a coreless silica fibre.
A convenient method of forming an optical device, and which constitutes a second aspect of the present invention, is one in which a block is fusion spliced to the end of the fibre waveguide after which the block is cleaved to provide a block of predetermined length.
The differential diameters of the block and the waveguide is limited by the fusion splicing process. If a longer block is required this limitation can be overcome by forming a stepped block by splicing two sub-blocks of increasing diameter together, the lesser diameter sub-block diameter being spliced to the optical waveguide.
One method of forming such an optical device is one in which two sub-blocks are first fusion spliced together. The one of lesser diameter is then cleaved to provide a first sub-block of predetermined length. This cleaved end is then fusion spliced to the waveguide and the sub-block of greater diameter subsequently cleaved to provide a second sub-block of predetermined length.
Devices with blocks having more than two sub-blocks similarly fusion-spliced to form a single block could also be used with the present invention.
An alternative method of providing a block having a larger diameter remote from the waveguide is to use a block in the form of a tapered rod, for example, of frustroconical shape. The angle of divergence of the sides of the block should be selected to avoid unwanted reflections back in the waveguide.
In some cases, if the cladding of the waveguide is sufficiently wide, one could use a block which falls within the perimeter of the waveguide.